Process for stretch-reducing tubes



Feb. 24, 1970 F. KOCKS 3,496,745

PROCESS FOR STRETCH-REDUCING TUBES Filed May 1. 1967 United States Patent 3,496,745 PROCESS FOR STRETCH-REDUCING TUBES Friedrich Kocks, Dusseldorf, Germany, assignor to Firma Friedrich Kocks, Dusseldorf, Germany Filed May 1, 1967, Ser. No. 635,152 Int. Cl. 1321b 37/02 US. CI. 72-16 6 Claims ABSTRACT OF THE DISCLOSURE A process for stretch reducing tubes in a multi-stand reduction rolling mill which produces a variable degree of stretch comprises measuring the average initial wall thickness of the tube, for example by measuring its electrical resistance, its length and its outside diameter, and adjusting the average degree of total stretch produced by the mill in accordance with this average measured thickness and the magnitude of the reduction required, making measurements of the individual deviations of the wall thickness of the tube from the average measured thickness, for example by measuring the penetration of 'y-rays through the wall, and feeding these measurements in the form of control signals to an automatic control device which changes the degree of stretch produced by the mill in proportion to the measurements by changing the speed of a driving motor which drives one or more stands of the mill.

This invention relates to a process for stretch reducing tubes by means of a reduction rolling mill in which the degree of stretch produced can be varied. The purpose of the arrangement according to the invention is to ensure that all the stretch reduced tubes produced from a given batch of tubes have a greater uniformity of wall thickness, and if necessary to ensure at the same time that each individual stretch reduced tube has a more uniform wall thickness over its length, than has hitherto been obtainable by the customary methods. In the process of stretch reducing tubes by means of multi-stand reduction rolling mills, the starting material can be either rough pierced tube blanks, which can be made in a variety of ways, or lengths of tube made from strip by longitudinal seam welding. For various technical reasons it has proved to be impossible to prepare large quantities of pierced blanks or seam welded lengths of tube in such a way that all the tubes have the same wall thickness, or that each tube has a uniform wall thickness over its entire length. For example seamless tubing drawn on a push bench varies in wall thickness because the mandrel bars of a particular batch in the first place differ in diameter as a result of the manufacturing process, and in the second place acquire differences in diameter by wearing to different degrees during use. Moreover the starting pierced blanks often have diameters which vary along the length of the blank, mainly because unevenly heated pieces have been rolled.

In the process of finish rolling tubes on multi-stand reduction rolling mills in which the degree of stretch can be varied, differences in wall thickness can be compensated in principle by adjusting the degree of stretch. However this possibility has hitherto been exploited by first of all measuring the wall thicknesses of the finished tubes, and then adjusting the degree of stretch given by the rolling mill on the basis of the observed deviations from the specified wall thickness. This method cannot give finished tubes all having the same wall thickness, nor a uniform thickness over the length of each tube, in the first place because the tubes which have been measured are already finished products and can no longer be changed, and in the second place because it is not possible to predict in this way whether the material subsequently moving through the rolling mill really does require the adjustment made to the degree of stretch, and in particular whether all the individual pieces require this particular adjustment. The process according to the present invention allows any desired number of tubes to be manufactured all having the same finished wall thickness and each tube having a uniform wall thickness over its entire length. Furthermore the process is applied using a customary multi-stand reduction rolling mill giving a variable degree of stretch. The process is applicable both to the production of finished tubing of specified dimensions, and to the production of semi-finished tubing.

According to the invention, a process for stretch reducing tubes in a multi-stand reduction rolling mill which produces a vairable degree of stretch comprises measuring the average initial wall thickness of the tube and adjusting the average degree of total stretch produced by the mill in accordance with this average measured thickness and the magnitude of the reduction required, making measurements of the individual deviations from the average measured thickness, and feeding these measurements in the form of control signals to an automatic control device which changes the degree of stretch produced by the mill in proportion to the measurements.

In this way the degree of stretch given by the mill is increased when the initial wall thickness is too great and decreased when the initial wall thickness is too little.

The process is applicable to the manufacturing of both seamless and welded tube, irrespective of whether the reduction rolling mill is operating by itself or in conjunction with a tube welding machine and/or further machines.

With regard to methods of measurement, various methods are known for measuring wall thickness in sheet material and tubes. These known methods satisfy the requirement in practice, in regard to accuracy and in that they produce results which can be effectively evaluated. For example the wall thickness of a tube can be measured by putting an electric coil around it and passing through the coil current at medium frequency. The current flowing through the coil, or the power absorbed by the coil can be measured. Alternatively a strip or starting tube can be irradiated with 'y-rays and the penetration measured. Wall thickness can also be determined indirectly, for example by measuring the electrical resistance of a length of tube in a circuit between two feeler rollers, the length of tube forming one arm of a Wheatstone bridge. These methods allow measurements to be made continuously of entire starting lengths of tube, and if desired of predetermined longitudinal portions of the tubes.

Furthermore it is possible to calculate the average wall thickness of a pierced tube blank very accurately from the Weight, the length and the diameter. But this method does not allow determination of individual or systematic local variations in wall thickness. The calculation of average wall thickness from weight is therefore preferably used in the manufacture of comparatively short seamless tube, the same adjustment of degree of stretch being in this case used for all the pierced blanks. The method eliminates differences in the average wall thicknesses of the finished tubes. On the other hand, the process of rolling seamless tube in the preliminary rolling train often produces wall thicknesses which increase by several tenths of a millimetre from the leading end to the trailing end of the tube. Wall thickness variations of this kind are eliminated, according to a further feature of the invention, by adjusting the degree of stretch given by the reduction rolling mill in such a way that the adjustment varies systematically between the two ends of the rolled tube. This regularly repeated change of adjustment produces an excellent finished product because, as has been found in practice, rough pierced tube blanks which have passed through the same preliminary rolling train under unvarying operating conditions all show the same variation in wall thickness, over the length of the piece of tube, during long periods of operation.

The process according to the invention will now be described in greater detail with reference to an example:

The starting material consists of pieces of tube Whose wall thickness varies from piece to piece, and also varies between the two ends of each piece of tube. The requirement is to stretch roll the tube in a reduction rolling mill, at rolling temperature, in such a way that the wall thickness difierences are eliminated and the finished tubes all have the same wall thickness, the wall thickness also being substantially uniform over the length of each piece. The rolling mill has a central speed control allowing adjustment of the degree of stretch applied. The degree of stretch applied to the tube is proportional to the speed of the control motor controlling the mill. Furthermore the reduction rolling mill is also equipped with a photocell which responds to the heat radiated from the tube passing through the mill. The photocell is situated after approximately the first third of the roll stands in operation, but if the mill drive is very slow to respond to required changes, or if the instrumentation involves a considerable time of delay, the photocell can be situated near the first roll stand, or even in front of it.

Before the first piece of tube is fed into the furnace the average Wall thickness of each piece of tube is determined, for example calculated from the length and the electrical resistance, or from the dimensions and the weight, as described above. The values thus determined are adjusted on the basis of the wall thickness specified for the finished tubing and the adjusted values are converted to give the control motor speed required for each piece of tube. This value is stored in the control system.

As soon as the leading end of the first tube enters the rolling mill the photocell responds and, acting through the automatic control system, starts the control motor, which adjusts the degree of stretch on the basis of the control value first stored. As soon as the trailing end of the piece of tube passes the photocell the control motor is stopped. This process is repeated each time a piece of tube passes through the mill. The pieces pass through in the sequence of the measuerd values stored in the automatic control system. The operations proceed as described, irrespective of Whether the preceding tube has or has not entirely left the mill before the following tube has entered the first roll stand.

For adjusting the speed in accordance with the stored information a field voltage potentiometer can be used,

for example, acting on the control motor of the reduction rolling mill, or on its Leonard generator, the sliding contact of the potentiometer being actuated through a motor drive by the automatic control system.

As soon as the trailing end of a piece of tube has passed beyond the photocell the sliding contact of the field voltage potentiometer is rapidly returned to its initial position. When the leading edge of the next piece of tube acts on the photocell the control motor is adjusted to the next stored speed.

Furthermore, the sliding contact of the potentiometer can be continuously moved so that the control motor is continuously influenced and continuously changes the degree of stretching during the passage of a piece of tube through the mill. Moreover by varying the rapidity of response of the potentiometer slider, that is to say the speed of movement of the sliding contact, allowance can be made for the number of roll stands in operation. In this way starting tube whose wall thickness varies along the length of the tube can be processed to give finished tubes of uniform wall thickness all along the piece.

An example plant working according to the invention is schematically shown in the illustration. The plant comprises a reducing rolling mill on which a tube having the dimensions as at A is reduced to the dimensions as at B.

The reduction of diameter is effected by rolls, at least two of which form one of the roll passes 1 to 21. According to which diameter shall be obtained at B, more or less roll passes are available, which are arranged in a narrow distance one after the other forming a decreasing passage for the rolling stock. The rolls are driven by a gear drive 23 in such a manner, that their speeds in crease in rolling direction (arrow 22) from roll pass to roll pass as the cross-section diminishes. Whereas the diameter reduction of the tube is determined by the roll passes only, a change of the wall thickness at the tube can be obtained in addition to this by changing the difference of roll speeds between contiguous roll passes. For obtaining e.g. a reduced wall thickness, the roll speed differences must be increased, and this increase has to intensify in rolling direction from roll pass to roll pass, when the speed of the rolls of the first roll pass remains approximately unchanged and the wall thickness of the starting tube remains exactly unchanged.

Likewise, the speed differences have to be changed, when a reduced wall thickness of the finished tube shall not be obtained, but this shall remain uniform, rolling, however, being started from a wall thickness increased against the preceding rolling.

Facilities for reducing mills are known with the aid of which such variations of speed differences can be made between all roll passes commonly and by means of which, as it is said, the degree of stretch of the rolling mill is changed.

In the present instance, a nominal degree of stretch is according to the illustration set as voltage at 25 for the regulator 24 on the righthand side in the picture.

The plant is provided according to the illustration with an isotopes ray-penetration device 26 for measuring the wall thickness of the tube with dimensions as at A. The electrical measuring voltage of the counting tube 27 is transmitted as actual value of the wall thickness to the regulator 28 on the left-hand side in the picture and compared in same with a nominal value of wall thickness set as voltage at 29. The regulator 28 influences in the measure of deviation of the measured Wall thickness from the nominal wall thickness upon the regulator 24 of the degree of stretch.

Supposed: a tube at A with a nominal wall thickness of 4 mm. shall be reduced to a tube at B with a wall thickness of 3 mm.

In case the actual and nominal wall thicknesses are equal, the setting of the nominal degree of stretch in the regulator 24 remains uninfiuenced. When on the contrary an actual wall thickness of assumed 4.2 mm. has been measured and compared in the regulator 28 with the nominal wall thickness set at 29, the left-hand regulator 28 gives a signal to regulator 24 corresponding to the deviation of the compared wall thicknesses and converted in consideration of the quantity of the used roll passes in variation of the degree of stretch. In the right-hand regulator 24 the signal is superimposed to the nominal degree of stretch. The degree of stretch corrected in such a manner is then given to the gear drive 23 at 30. For the assumed deviation of Wall thickness of 0.2 mm.=5%, the corrected nominal degree of stretch has to be greater than the nominal degree of stretch set in advance.

The transmission of the deviation of the wall thickness from one regulator to the other only takes place, when a tube is in the range of the ray-penetration facility. When the rolling process is interrupted, the transmission of the deviation is also interrupted and the actual degree of stretch is regulated alone according to the nominal degree of stretch, i.e. it is in the preselected normal position.

I claim:

1. A process for stretch reducing tubes in a multistand reduction rolling mill which produces a variable degree of stretch, comprising the steps of adjusting the average degree of total stretch produced by said mill in accordance with an initial average measured thickness of a tube to be stretch reduced and in accordance with the magnitude of the reduction required, making measurements of the individual deviations of the wall thickness of said tube from said average measured thickness and feeding said measurements in the form of control signals to an automatic control device which changes the degree of stretch produced by said mill in proportion to said measurements.

2. A process as claimed in claim 1, including the steps of making measurements of the individual deviations from the average measured wall thickness of a batch of tubes, storing said measurements and subsequently feeding said tubes in said batch to said rolling mill in sequence and feeding said stored measurements to said automatic control device in the same sequence as said tubes are fed to said rolling mill whereby said automatic control device changes the degree of stretch appropriately as each tube in said batch passes through said mill.

3. A process as claimed in claim 1, including the steps of making measurements of the individual deviations from said average well thickness along the length of said tube and feeding said measurements to said automatic control device when said tube enters said mill to cause said automatic control device to change the degree of stretch continuously.

4. A process as claimed in claim 1, including the step of measuring said wall thickness deviations by measuring the penetration of rays through the wall of said tube.

5. A process as claimed in claim 1, including the step of determining said average wall thickness of said tube by measuring the electrical resistance, the length and the outside diameter of said tube.

6. A process as claimed in claim 1, including the step of determining the average wall thickness of said tube by measuring the weight, the length and the outside diameter of said tube.

References Cited UNITED STATES PATENTS 2,949,799 8/1960 Walker 72-16 2,972,269 2/1961 Wallace et al. 7216 3,030,836 4/1962 Gochenour 7216 3,109,095 10/1963 Van Horne 25083.3

MILTON S. MEHR, Primary Examiner 

